Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation (e.g., visual or near infrared radiation) by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a visible record of the electrostatic image.
Photoconductive elements, also called photoreceptors, are composed of an electrically conductive support and at least one active layer which is insulating in the dark but which becomes conductive upon exposure to light. The support may be in one of many forms, for example, a drum, a web or belt, or a plate. The photoreceptor can comprise one or multiple active layers. The active layer(s) typically contains one or more materials capable of the photogeneration of charge carriers (electrons or holes) and one or more materials capable of transport of the generated charge carriers.
Numerous materials have been described as being useful components of the photoreceptor. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines, dye-polymer aggregates, and the like. Organic compounds are particularly useful for several reasons: they can be prepared as flexible layers; they have spectral sensitivities that can extend throughout the visible and into the near infrared regions of the spectrum; and they can be readily prepared by low cost solvent coating technologies. Photoconductive elements prepared from organic materials are known as organic photoconductors (OPCs).
OPCs can be prepared with single or multiple active layers. In most OPCs, charge transport occurs through movement of a single type of charge carrier, electrons or holes, but not both. When only one carrier is mobile, trapped carriers of opposite sign can be created, resulting in a change in sensitometry of the active layer with successive cycles and in a phenomenon known as latent image hysteresis. One solution to the problem of latent image hysteresis is to separate the charge generation and transport functions into separate layers, referred to as the charge generation (CGL) and charge transport (CTL) layers, to form a dual or multi-layer photoconductive element. These elements offer additional advantages of improved process lifetimes that make them useful in high volume copying and printing applications. A disadvantage of the multiple layer architecture is that only one polarity of surface potential may be employed, limiting the electrophotographic processes in which the element can be used.
There are certain restrictions on the charge generation (CGM) and charge transport (CTM) materials that can be combined to form a useful photo-conductive element. In particular, the useful combinations of CGMs and CTMs are limited by their relative oxidation or reduction potentials. The oxidation potential is the relevant parameter in the case of hole transport; the reduction potential is the relevant parameter for electron transport. It is only when an appropriate match of potentials between the CGM and CTM is achieved that a useful photoconductive element can be prepared. Thus, while it is possible to demonstrate a material is capable of charge transport, for example, this does not necessarily imply that this material would be useful in a photoconductive element used in electrophotography.
Many useful photoconductive elements containing appropriately matched CGMs and CTMs are known. Most of these are comprised of CTMs which are capable of transporting only a single carrier type, either electrons or holes; these elements are referred to as monopolar photoconductive elements. These elements have the limitation that they can only be used with one polarity of charging, limiting the processes in which they can be applied.
Few bipolar photoconductive elements, comprised of CTMs capable of transporting both carrier types, are known. The known elements have several disadvantages. The bipolar transport in these elements arises from CTMs which are complexes of at least two separate materials. The necessity to form the complexes both increases the complexity of the manufacture of these elements and increases the likelihood that imperfectly formed complexes will give rise to defect points in the element. Also, the complexes are not stable over long periods of time. Thus, there is a need for a bipolar photoconductive element that is comprised of a single CTM capable of bipolar transport.
Only one bipolar charge transport molecule, a single molecule capable of transporting both electrons and holes, is known. Murray et al. (B. J. Murray, J. E. Kaeding, W. T. Gruenbaum, and P. M. Borsenberger, Jpn. J. Appl. Phys. 1996, 35, 5384-5388) reported that N-(p-(di-p-tolylamino)phenyl)-N'-(1,2-dimethylpropyl)-1,4,5,8-naphthalenet etracarboxylic diimide (TAND), in combination with an amorphous selenium CGL, could transport both electrons and holes. Amnorphous selenium is undesirable for practical application in a photoconductive element because of the hazards associated with its deposition and disposal. Further, its spectral sensitivity extends to only 500 nm, too low for most applications. Kaeding et al. (J. E. Kaeding, B. J. Murray, W. T. Gruenbaum, and P. M. Borsenberger, J. Imag. Sci. Technol. 1996, 40, 245-248) further reported bipolar transport by TAND in combination with an unspecified perylene diimide. While the electron and hole mobilities of TAND are analyzed in this paper, no mention is made of the electrophotographic properties of a photoconductive element containing TAND in combination with a CGL that would be useful in an electrophotographic process. The information in this paper is insufficient to prepare a practical photoconductive element for use in an electrophotographic process.
In order to be useful in an electrophotographic process, a photo-conductive element must display good photosensitivity and low residual voltage after exposure. The photosensitivity is a measure of the amount of energy required to discharge the photoconductor from an initial voltage to some predetermined potential. The residual voltage (V.sub.r) is a measure of the charge remaining on the element after exposing the element. The residual voltage is the minimum voltage to which a photoconductive element can be discharged. A high residual voltage can give rise to a lower potential difference between charged and discharged areas of the element on subsequent imaging cycles. Blurred, fogged, or incomplete images result. Hence, for high process efficiency, high photosensitivity and low residual voltage are desired.
The electrophotographic properties of a photoconductive element are not inherent to a particular CTM or CGM but arise from the combination of CTM and CGM used to prepare the photoconductive element. Thus, materials which are known to be capable of charge generation or of charge transport will not obviously be useful for a photoconductive element used in an electrophotographic process.
A useful bipolar photoconductive element must display the desirable electrophotographic properties under both polarities of initial surface charge. A bipolar photoconductive element can be defined by the ratio of the exposure energies measured under positive and negative polarity initial charging. Specifically, if the exposure energy of the element after positive polarity charging is denoted E.sub.50%.sup.+ and if the exposure energy of the element after negative polarity charging is denoted E.sub.50%.sup.-, then a bipolar photoconductive element is one in which .alpha.=E.sub.50%.sup.+ /E.sub.50%.sup.-, where .alpha. is between 0.25 and 4.0. Both E.sub.50%.sup.+ and E.sub.50%.sup.- are measured in erg/cm.sup.2 and measure the energy necessary to discharge the photoconductive element from 400 V to 200 V.